Mercury just wanted to look its best. As the years rolled by the planet was losing the fire of its youth, but it was also slimming down, shedding a few pesky kilometres from its round, rocky body. Then the wrinkles started to sprout…

When the solar system formed about 4.6 billion years ago, Mercury was a hot ball of molten material. The tiny planet cooled quickly, shrinking in size and causing its relatively thin crust to crumple up. The shrinking seems to have slowed down after about a billion years, but the grey, pockmarked world we see today is crisscrossed by steep crustal ridges that reveal the period of rapid contraction.

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Astronomers first noticed these ridges in pictures of Mercury taken by the Mariner 10 spacecraft in the 1970s. Based on their lengths and heights, they calculated that the roughly 5000-kilometre-wide planet shrank by about 3 kilometres, on average. But Mariner 10 mapped only about 40 per cent of the planet’s surface during its three flybys.

NASA’s Messenger spacecraft, which has been orbiting Mercury since 2011, has been able to map the entire planet and has snapped images of even more ridges. Using the probe’s latest data, Paul Byrne at the Carnegie Institution in Washington DC has found that Mercury shrank more than previously thought, losing about 11 kilometres from its original girth.

Smooth move

This intense squeezing of the crust should have made a tight seal that would have prevented any magma from leaking to the surface through volcanism. But in 2008, Messenger also discovered smooth plains indicative of ancient lava floods.

Using Messenger data, Christian Klimczak, also at the Carnegie Institution, and his colleagues have mapped the lava flows and figured out the ages of the surrounding surfaces. They found that volcanism overlapped with rapid contraction for roughly 200 to 300 million years.

At a meeting of the American Geophysical Union in San Francisco in December 2013, Klimczak said meteor impacts may be the best explanation for the unexpected volcanism. The hits could have released built-up pressure in the compressed crust and let lava pour out while the planet was shrinking. In fact, images of Mercury show several lava flows inside large craters, supporting this idea.

Shrinking moon

A similar process may be what created our moon’s dark volcanic plains, called maria, such as the Sea of Tranquility. Like Mercury, the moon also contracted as it cooled and is covered by impact craters, many of which are filled with ancient lava flows.

After analysing the moon’s wrinkled surface with data from the Lunar Reconnaissance Orbiter, Klimczak thinks that the moon shrank by at least 2 kilometres – previous estimates pegged the contraction at only about 200 metres.

The results of these studies have been a major contribution to our understanding of volcanic activity across the solar system, says Clark Chapman of the Southwest Research Institute in Boulder, Colorado. “This linkage of basin-forming impacts and volcanism is an important insight in planetary geology and geophysics.”

Update&colon; This article was first published on 2 January 2014 following work presented at the American Geophysical Union meeting in December 2013. It has now been updated to reflect publication of some of that work on 16 March 2014 (Nature Geoscience, DOI&colon; 10.1038/ngeo2097).